You can power them up that is a good use for them, but also you get paid interest just for holding them. Plus as @alexc points out you can use as currency between other Steemers. The advantage of this is that you can buy Steem at a better rate than an outside exchange. In time you'll have some serious leverage both on and off the site.

Oh and Blocktrades say that they'll have a way to convert SD to BTC in a few days, once a market is established, that will prove very useful.

CG